An example of conventional plasma display panels (hereinafter referred to as PDPs) is shown in FIG. 12, a perspective sectional view of a part of a conventional AC PDP.
As shown in FIG. 12, the AC PDP comprises a front substrate 305 and a back substrate 315. The front substrate 305 is such that a plurality of pairs of line-shaped scanning electrodes 301 and sustaining electrodes 302 are disposed in parallel on a transparent first glass substrate 300 (insulating substrate), and a dielectric layer 303 and a protective layer 304 are laminated over the electrodes. The back substrate 315 is such that a plurality of line-shaped data electrodes 311, positioned perpendicular to the scanning electrodes 301 and the sustaining electrodes 302, are disposed on a second glass substrate 310 (insulating substrate), a dielectric layer 312 is disposed over the data electrodes 311, barrier ribs 313 are disposed on the dielectric layer 312 in parallel lines so as to sandwich the data electrodes 311 therebetween, and phosphor layers 314 each having each color are mounted between the barrier ribs 313 along side walls thereof.
In a gap between the front substrate 305 and the back substrate 315, a rare gas, which is at least one of helium, neon, argon, krypton, and xenon, is enclosed as discharge gas, so as to form light emitting cells (or discharging spaces) 320 at open spaces where the scanning electrodes 301 and the sustaining electrodes 302 and the data electrodes 311 intersect each other in the gap in which is the gas is enclosed.
The scanning electrodes 301 and the sustaining electrodes 302 each are made of line-shaped conductive transparent electrodes 301a and 302a respectively in addition to bus electrodes 301b and 302b formed thereon respectively. The bus electrodes 301b and 302b contain silver (Ag), and are line-shaped and thinner than the transparent electrodes 301a and 302a. The data electrodes 311 also contain Ag.
The AC PDP is operated as follows. During a drive sustaining period after an initialization period and an address period, a pulse voltage is applied to the scanning electrodes 301 and the sustaining electrodes alternately. Then a sustaining discharge is caused in the discharging space 320 by the electric field generated between two parts on a surface of the protective layer 304 above the scanning electrodes 301 and above the sustaining electrodes 302, with the dielectric layer 303 interposed between the electrodes and the protective layer 304. Ultra-violet ray emitted by the sustained discharge excites phosphors in the phosphor layers 314, and visible light from the phosphor layers 314 is used for display light.
Here, a process for forming the scanning electrodes 301, the sustaining electrodes 302, the dielectric layer 303 and the protective layer 304 formed on the first glass substrate is briefly explained. First, the line-shaped conductive transparent electrodes 301a and 302a made of tin oxide or indium tin oxide (ITO) are formed on the first glass substrate 300. By patterning and baking a photosensitive paste containing Ag over the transparent electrodes using photolithography, the line-shaped bus electrodes 301b and 302b containing Ag are formed. Further, the dielectric layer 303 is formed by printing and baking a dielectric glass paste. Finally, the protective layer 304 is formed by evaporating magnesium oxide (MgO).
Next, a method for forming the data electrodes 311, the dielectric layer 312, the barrier ribs 313 and the phosphor layers 314 formed on the second glass substrate is briefly explained. First, by performing a photolithography method to the photosensitive paste containing Ag and baking the same, the line-shaped data electrodes 311 containing Ag are formed on the second glass substrate.
Then, by printing and baking a dielectric glass paste over the data electrodes 311, the dielectric layer 312 is formed. Further, the barrier ribs 313 are formed using a screen printing method, a photolithography method, and the like, and after that, the phosphor layers 314 are formed using such a method like a screen printing method and an ink-jet method.
Finally, the front substrate 305 and the back substrate 315, each obtained in the above stated process, are attached together (sealing) in a manner that a sealing glass interposed therebetween at the circumference of the substrates are molten and cooled down, and then exhausting air and enclosing a rare gas are done, and thus the panel is formed.
Next, a more specific explanation about the method for forming the bus electrodes 301b and 302b and the data electrodes 311 by the photolithography method using the Ag photosensitive paste is given below.
First, by applying the Ag photosensitive paste uniformly using the printing and the like method, an Ag photosensitive paste layer is formed on the first glass substrate 300 to which ITO is evaporated. Then, dry treatment is performed so as to remove a solution from the Ag photosensitive paste layer.
Next, by irradiating ultra-violet ray through a photomask, an exposed part and un-exposed part are formed on the Ag photosensitive paste layer corresponding to an electrode patterns. The exposed part later forms a pattern for the bus electrodes.
Further, the exposed part is fixed on the first glass substrate 300 by performing a developing treatment.
Finally, by performing baking treatment, pre-baked electrodes are made into the bus electrodes.
As have been explained in the above, in a case where the patterning is carried out using the photolithography method to the Ag photosensitive paste, the baking treatment is always performed after the patterning in order to burn the resin component in the paste, and it has been noted as a problem that an edge-curl is caused in this process. The edge-curl is considered to be caused mainly by an effect of tensile force during heating.
The edge-curl is a phenomenon in which the side edges of the pre-baked bus electrodes camber upward of the first glass substrate after baking. When the edge-curl occurs, it becomes difficult to form the dielectric layer over the bus electrodes. In addition, a surface angle of side edges after baking could become very sharp. Because an electric field concentrates at the sharp edges in driving the panel, the dielectric layer formed so as to cover the electrodes becomes susceptible to dielectric breakdown. For this reason, a surface of the side edges of the bus electrodes and the data electrodes are polished after baking in some cases, so as to make the side edges obtuse.
It has also been noted as a problem that, because light reflectivity of silver material is relatively large, contrast in the display light emission is drastically deteriorated when the bus electrodes on the front substrate are made of material containing Ag as explained above due to incident light to a surface of the front substrate reflected by the bus electrodes. For this reason, the bus electrodes having an optical bilayer structure, a composite lamination in which two metal layers each containing black pigment and silver respectively are laminated in a stated order on the first glass substrates (hereinafter referred to as a “black and white composite lamination”), is put into practical use as the bus electrodes disposed on the front substrate.
Such bus electrodes having the bilayer structure are also formed using the photolithography method as in the case of the electrodes having one layer as explained above.
More specifically, a first printed layer is formed by applying a photosensitive paste containing black pigment. Next, the paste is dried so as to remove a solution from the first printed layer.
Then, a second printed layer is formed by applying an Ag photosensitive paste on the first printed layer. Further, the first and second printed layers are dried so as to remove solutions from the both layers.
Next, by irradiating ultra-violet ray through a photomask, an exposed part and an unexposed part corresponding to an electrode pattern are formed on the first and second printed layers. Usually, the exposed part later forms a pattern for a black and white composite lamination.
After this, the exposed part is fixed to the first glass substrate by developing.
Then, by baking, the laminated layers of black pigment and Ag become the black and white composite lamination.
In the forming process, the side edges of the black and white composite lamination could also camber upward (edge-curl). Accordingly, a sectional surface of the black and white composite lamination in a widthwise direction becomes concave, and the side edges sharp surface angles in some cases.